Spinal fusion is defined as the joining together of two or more adjacent vertebrae through a bridge of bone for the purpose of eliminating motion between those vertebrae. One specific type of spinal fusion is known to those skilled in the art as interbody fusion and consists of fusing the adjacent vertebrae through the disc space (the space previously occupied by the spinal disc interposed between the adjacent vertebral bodies). When such a procedure is performed from the anterior aspect of the spine (from the front) that procedure is known as anterior interbody fusion.
Typically, bone grafts are placed into the disc space to position the vertebrae apart so as to create more space for the nerves, to restore the angular relationship between said adjacent vertebrae to be fused, and to provide for material that can participate in and promote the fusion process. Substrates, other than bone, such as hydroxyapatite and/or artificial spinal fusion implants may also be used.
In general the ability to achieve bone fusion appears to be related to certain metabolic biochemical factors, the quality and quantity of the osteogenic material present (bone forming material, most commonly bone itself), the surface area available for the fusion to occur over, and the stability of the construct being fused (the ability to resist unwanted motion at the fusion site).
Consistent with the foregoing, it is desirable for the surgeon to place the largest fusion implant, generally bone, within the disc space as this provides for both the greatest surface area, and fusion mass. Furthermore, the greater the area of contact, the greater the stability of the bone-graft construct, such, that the graft is less likely to migrate, to itself collapse, or conversely to penetrate into the adjacent vertebrae as the forces across the fusion site are distributed over a greater area.
The disc space can best be described as having a biological rather than a geometric shape in that the adjacent vertebral endplate surfaces are complexly biconcave in portions, convex in others, while in still other areas extremely dense portions of bone project like xe2x80x9cpillarsxe2x80x9d almost perpendicularly from the plane of the vertebral endplates, thereby forming partial, but substantial, side walls about the posterolateral (toward the back and side) portions of the disc space, the latter being particularly pronounced in the cervical spine.
As the bone graft used for the purpose of interbody fusion must have sufficient structural integrity to support the superincumbent body weight and the forces present across the portion of the body in which the graft is inserted, generally only quite solid portions of bone can be used for this purpose. Such portions of bone can only be cut, such as with a saw, rather than molded to fit the disc space. Even for the most skilled surgeon, it is not possible to shape such grafts to precisely fit the complex contours of the vertebral endplates adjacent the disc space. Therefore, the bone grafts are generally considerably smaller in width and depth than the disc space itself so as to confine such grafts to the more relatively flat area located about the mid portion of the vertebral endplates The term xe2x80x9crelatively flatxe2x80x9d is a correct description of the mid portion of the endplate in that even this region of the vertebral endplate is not truly flat, such that it is relatively rare to achieve full congruency between the machined surface of the bone graft and the biologically determined shape of the vertebral endplate. This further compromises the quality of the fusion construct in that the area of contact between the vertebrae and the graft is suboptimal with a loss of both support area and correspondingly, graft and construct stability.
Further factors tending to limit the dimensions of the graft to less than that of the disc space include, for example in the cervical spine, the danger of the graft accidentally escaping the disc space laterally (to the side), damaging the vertebral arteries and causing a cerebral infarct and the danger of penetrating posteriorly (toward the back) and injuring the spinal cord causing paralysis. Furthermore, the previously described pillars of dense bone projecting from the overall plane of the vertebral endplates in the posterolateral portions of the disc space and commonly known as either uncinate processes or the joints of Luschka, tend to block-the lateral and posterior placement of the graft(s) and tend to confine and limit the placement of the graft(s) to the anterior and central portions of the disc space.
To achieve fusion, it is necessary to at least vigorously scrape the outermost layer of the vertebral endplates until bleeding occurs to encourage the fusion, which invokes a healing process of the bone. Since the vertebral endplates are generally quite strong, it is desirable to preserve this structure even while scraping into it which can not reliably be achieved by the means of the prior art. In the past, anterior interbody fusion would be performed by removing at least a portion of the intervertebral disc and then utilizing hand held, free-hand instruments including, but not limited to, osteotomes, chisels, curettes, rongeurs, and burrs to scrape and shape the vertebral endplates and vertebral bone stock, which operations would be performed generally by working on one vertebra at a time, and independent of the position of the adjacent vertebra.
As a final consideration, not only are the vertebral endplates complexly shaped, but so are the interposed discs themselves. That is, the vertebrae of the spine are generally aligned in curved, rather than straight patterns when viewed from the side. When such curves are convex forward, as they are in the cervical and lumbar spine, the vertebrae are said to be in lordosis. Such lorcosis may be the result of wedging of the vertebral bodies, of the discs, or a combination of both. When lordosis is the result of a generally wedge shaped disc, it has generally proven difficult to reliably restore that overall wedged shape to the disc space itself for the purpose of fusing the adjacent vertebrae with precisely the correct amount of lordosis.
While the discussion above has been in regard to anterior interbody fusion, it may be desirable to replace a damaged or diseased disc with a flexible member, or mechanical xe2x80x9cartificial discxe2x80x9d, in which situation maximizing the surface area and congruency of contact, and controlling the angular and spatial relationships of the vertebrae adjacent that disc space would still be of great importance. As to be discussed, the present invention pertains to a means and method for the preparation of the space between adjacent vertebral bodies (xe2x80x9cthe disc spacexe2x80x9d) suitable for the implantation of an artificial disc or for interbody spinal fusions.
Attempts have been made in the past to create a guided milling apparatus for use in surgery of such appendicular joints as that of the knee. For example, U.S. Pat. No. 5,486,180 issued to Dietz on Jan. 23, 1996 teaches the use of a guided milling apparatus. The Dietz apparatus is not capable of working in the spine to prepare a space between adjacent vertebral bodies and differs from the present invention in the following ways;
1) The Dietz apparatus requires that the bone be exposed end on (Col. 1, lines 34-36, Col. 2, lines 46-47, FIGS. 1,2, and 3). In the present invention, the xe2x80x9cendsxe2x80x9d of the vertebrae to be prepared are the vertebral endplates which can not be exposed on end except by dislocating the vertebrae which would cause the most grievous harm to the patient.
2) The Dietz apparatus is for the preparation of a single bone at a time (Col. 1, lines 34-36, Col. 1, lines 49-50; FIGS. 1,2,3,5,7).
3) The milling end of the Dietz apparatus removes the bone parallel to the template surface (Col. 4, lines 7-9, Col. 4, lines 50-53, FIGS. 5 and 7). In the spine, there is insufficient space available within the disc space interposed between adjacent vertebrae to insert, accommodate or operate the Dietz mechanism; this would be true regardless of actual size of the Dietz device for any size that would remain workable for use in the spine.
4) The Dietz apparatus in incapable of affixing the opposed bones on both sides of the joint simultaneously, or of preparing both joint surfaces with the opposed bones in fixed relationship with each other.
5) The Dietz apparatus teaches a means for cutting across two dimensions while controlling (fixing) for depth. (FIGS. 5, 27).
6) Dietz teaches that the mill end is too large to pass through the template guide surface so as to confine the mill end beneath the guide means. (Col. 3, lines 8-19, Col. 4, lines 24-53; FIGS. 5 and 7). This thus requires that the enlarged burr portion enters the bone not through its end or face, but rather on the front surface of the bone, which entrance occurs through a cut out slot, is deep to the guide plate, and with the burr spinning about an axis parallel to the longitudinal axis of the bone itself. (Col. 2 Line, 35-37, FIGS. 1 and 2).
7) The Dietz apparatus is limited to the cutting by use of a burr along a nonlinear path. (Col. 2, lines 65; Col. 3, lines 4-6; Col. 4, line 2, FIGS. 4 and 6). This is not arbitrary as the path of the burr is guided by either or both of a pivot, allowing only for a series of arcs, and/or a branched and serpentine slot system also configured to produce only a series of arcs. (Col. 2, lines 41-42, line 52, line 65 through Col. 3, line 4, and FIGS. 1, 2, 4 and 26).
There is therefore a need for a method and means for preparing the vertebral bodies and the vertebral endplates adjacent to a disc space to be fused by interbody fusion that.
1) allows for the safe preparation of the disc space to the optimal depth and width so as to allow for the correct use of the largest possible fusion implant which would be associated with the direct benefits of providing for the maximum mass of osteogenic material, the largest possible surface area for fusion to occur over, increased graft and construct stability secondary to the increased area of contact, and the greatest protection against implant collapse or penetration into the vertebral bodies from the distribution of the loads over the greatest surface area;
2) allows for the preparation of the vertebral endplates to a known and uniform surface configuration, which configuration can be matched by a corresponding surface of the fusion implant: thereby providing for the greatest possible interface congruity between the vertebral endplates and fusion implant, and providing for the optimal contact surface, enhanced fusion area, enhanced graft and construct stability, and decreased load per surface area;
3) allows for the restoration of the correct vertebral alignment by preparing the vertebral endplates in fixed relationship-to each other adjacent the disc space so as to three dimensionally shape the disc space-fusion implant site;
4) allows for an efficient and reliable means for scraping the central portions of the outer layer of the vertebral endplates without the danger of removing those structures entirely; and
5) allows for the extension of the fusion area into the extremely supportive and extremely dense bone of the posterior lateral regions of the disc space.
The present invention is directed to an apparatus and method for use in spinal surgery for creating a space of selected shape and dimensions across the disc space between two adjacent vertebrae of the spine. The present invention comprises an integrated system of instrumentation and a surgical method that is capable of placing the adjacent vertebrae to be operated upon in proper angular relationship to each other, of locking the vertebrae in that position, of simultaneously preparing both vertebral endplates for the procedure, be it fusion or disc replacement, and specifically the creation of a space of a known shape and dimensions. The foregoing is achieved by the use of a power milling apparatus such that all free hand motions are eliminated and the site size, shape, and configuration, as well as the extent of vertebral endplate resection are made predictable, precise and reproducible. The instrumentation of the present invention allows for the safe controlled and protected preparation of the disc space to the optimal depth and width. The present invention allows for implant(s) or bone grafts to be placed onto the area of dense bone at the posterior lateral aspects of the disc space. The present invention allows for the maximum stability of the graft/implant, as well as the construct, by providing for the greatest possible interface surface area and congruency between the graft/implant and each of the adjacent vertebrae.
The present invention further provides for increased stability by creating butted corners, posterolaterally, into which the graft/implant may be fitted which corners prevent further movement of the graft/implant either posteriorly or to either side of the created space. The present invention is capable of the uniform preparation (scraping) of the central portion of the vertebral endplate otherwise leaving that structure intact, if so desired. The present invention makes possible the preparation of the disc space to be prepared in anatomical angular conformation, and the disc space can be formed so as to correspond to the known shape of the graft/implant to be utilized.
In one embodiment, the apparatus of the present invention comprises a milling block having a front face configured for placement against a segment of the spine and having at least one aperture for accessing each of the two vertebral endplates of the vertebral adjacent the disc space. The adjacent vertebrae are placed in the appropriate spatial relationship to each other with a distractor means. Instrumentation is disclosed for selecting and calibrating the maximum safe penetration depth of the distractor means into the disc space and for locking the selected depth to prevent unwanted over penetration into the disc space. The distractor means may be part of the milling block or may be a separate member that is coupled to the milling block either directly or by a distractor holder.
Associated with the milling block is a bone removal means for removing a portion of bone from each of the vertebral endplates adjacent the disc space. The bone removal means is capable of accessing the vertebral endplates through the aperture in the milling block. The milling block is held firmly against a segment of the spine by securing means, such as prongs, pins, screws and the like, or by the distractor means itself inserted into the disc space in contact with the endplates of the adjacent vertebrae Instrumentation is disclosed for selecting, calibrating and limiting the penetration depth of the bone removal means into the disc space and/or vertebrae and to prevent unwanted over penetration into the disc apace and/or vertebrae. The bone removal means may be guided with an instrument guiding means located in the aperture of the milling block and in slideable relation to the milling block permitting slideable transverse motion and/or vertical motion during operation of the bone removal means to remove a portion of bone from the vertebral endplates adjacent to the disc space.
The following is a brief outline of the steps of the surgical method of the present invention describing the use of the specific instrumentation in regard to the preferred embodiment:
1. The area of the spine to be fused is exposed and a partial disectomy is performed, whereby a portion and preferably a large portion of the disc is removed while preserving the annulus fibrosis portion of the disc along at least both sides of the disc space.
2. The interspace so created is distracted and while not requisite, preferably to its optimal height, which height is determined by the known normal spatial relationships for that area the adjacent soft tissue structures. The interspace is then measured for height, depth, and width. The width of the interspace may be determined in reference to the inferior portion of the vertebral endplate of the superior vertebrae, and this determines the selection of the appropriate width for the milling block. The measured depth of the interspace, that is the distance between the front and back of vertebrae, will determine the selection of a distractor and milling means of slightly lesser depth. The height and depth of the interspace will determine the selection of the appropriate height and length of the distractor element, the shape of which is determined by both the need to either maintain or restore lordosis, as well as the shape of the implant which may or may not be wedged.
3. The correct distractor element is selected, having either a known fixed length, or preferably is adjustable and its optimal fixed length adjusted using a calibration gauge, integral markings or similar means.
4. The distractor apparatus is then attached to the milling block which has already been selected for the correct width.
5. The combined distractor apparatus and milling block assembly is then brought to the fusion site and the distractor element is introduced into the disc space. The distractor element may be introduced into the disc space turned on its side so as to facilitate introduction and then turned 90 degrees to distract the space or the distractor element may be introduced perpendicular to the plane of the disc space relying on its bullet-shaped leading edge portion to distract the vertebrae apart. The angular relationship of the two vertebral bodies adjacent that disc space will then be determined by the shape of the distractor element. It is appreciated that while not preferred, a distractor could be inserted into the disc space first, then the milling block assembly is brought into place relative to the spine thereafter.
6. The milling block is then secured to the anterior aspect of the spine preferably, by engaging each of the adjacent vertebrae.
7. The width and depth of bone resection may then be easily confirmed visually prior to any actual bone resection.
8. The distractor element and distractor apparatus are removed from the disc space.
9. The proper dimensioned bone removal means, corresponding to the previously employed distractor element, is selected and using the receiving depth gauge, the bone removal means is adjusted for depth and locked.
10. The bone removal means is secured to the milling port of the milling block, and the space is then milled to remove a portion of bone from the endplates adjacent to the disc space.
11. The milling apparatus is removed and the prepared space may be irrigated and sectioned through the milling block, or alternatively the entire milling assembly including the milling block may first be removed and the prepared space then irrigated and sectioned.
12. The prepared space is distracted utilizing conventional means and the appropriate implant or implants are then inserted into the prepared space.
In the alternative, the surgical method of the present invention may be performed by utilizing a separable milling block having a separable member with bilateral, lateral distractor elements such that the distractor elements are left in place in the disc space while steps 9-12 above of the method are performed. However, in this instance no distraction would be necessary in step 12 as the distractor elements remain in place while the appropriate implant(s) are inserted into the prepared space and the distractor elements are subsequently removed. The placement of bilateral distractor elements also provides a safety means for preventing the bone removal means and implant from exiting from the sides of the: disc space and out of the spine.
It is an object of the present invention to provide for a surgical method and instrument means for performing interbody spinal fusion or in the alternative of inserting an xe2x80x9cartificial disc implantxe2x80x9d for the purpose of maximizing the width and optimizing the depth of the disc and the bone removed from front to back, or back to front, from the vertebral endplates adjacent the disc space to be fused or implanted while confining such bone resection safely within the lateral, anterior (front) and posterior (back) limits of the disc space.
It is another object of the present invention to provide for a surgical method and instrument means for performing interbody spinal fusion or xe2x80x9cartificial discxe2x80x9d, implantation that provides for the rapid creation of both a known surface contour of each of the vertebral endplates adjacent a disc space as well as a known and reproducible shape of the fusion or implantation site itself.
It is another object of the present invention to provide for a surgical method and instrument means for performing interbody spinal fusion that allows for the utilization of a larger interbody spinal fusion implant(s) than was possible with the prior art, such an implant having the capacity for providing increased amounts of osteogenic material, increased surface area, increased area of contact, increased stability and the ability to provide for greater support through the fusion area.
It is another object of the present invention to provide for a surgical method and instrumentation for performing the preparation of the space between adjacent vertebrae for the purpose of implanting an artificial disc or fusion implant(s) having the optimal cross sectional area of contact with said adjacent vertebrae and where said cross sectional area may be as large as possible while remaining safely within the perimeter of the endplates of the adjacent vertebrae.
These and other objects of the present invention will become apparent from a review of the accompanying drawings and the detailed description of the drawings.